The advent of high resolution color photocopy equipment simplifies the task of currency replication to the point where it is becoming a crime of opportunity. Whereas such counterfeiting in the past was usually undertaken by skilled artisans perpetrating a deliberate criminal act, it is now becoming a simple process tempting the public to become casual counterfeiters.
U.S. Pat. Nos. 4,652,015 and 4,761,205 both describe techniques whereby a plastic strip containing metal characters is integrally-formed within the currency paper during the papermaking process to provide a "security thread".
The security thread remains virtually undetected under reflected light while being readably discerned with transmitted light which effectively defeats replication by any photocopy process.
When currency paper is printed by the intaglio process, the calendaring effect reduces opacity and thereby the hiding power of the paper fibers. Under careful post print inspection in reflected light, the metal characters appear brighter and lighter than the surrounding paper thus becoming legible.
It is believed that the presence of the light colored characters may be relied upon by the general public to indicate the presence of a security thread without further verification with transmitted light. A counterfeiter could then presumably duplicate the light characters with white toner to give the erroneous impression that a security thread is present.
An early attempt to eliminate the light characters by pigmentation of the plastic substrate strip was not totally successful since the outline of the pigmented plastic strip could be detected upon close scrutiny as a faint continuous line.
U.S. Pat. No. 4,398,994 describes a demetallization process for providing metal characters on a plastic substrate whereby a pigmented coating is selectively applied to the exposed surface of the metal characters. The surface of the metal characters facing the plastic substrate remains reflective. U.S. Pat. No. 4,242,378 teaches a method for coating the plastic substrate under the metal characters with a pigmented coating while leaving the exposed surface of the metal characters uncoated. For security devices fabricated in accordance with the teachings of these Patents the metal characters are discernible from either one surface of the paper or the other depending upon which side of the paper has the bare metal surface outward.
One purpose of the instant invention accordingly, is to provide a security paper containing a metallized security thread that is virtually invisible when viewed under reflected light from both sides of the paper yet is clearly visible from either side of the paper when viewed with transmitted light.